The demand of wireless data traffic is increasing due to the increasing popularity of data hungry applications, such as real-time video calls and video streaming. Data hungry applications have been enabled by the advances in hardware of the mobile devices such as mobile phones, tablets and laptops. In order to meet this high data demand, deployment of low power base stations, such as picocells, microcells, and femtocells, as well as distributed antennas, is becoming popular. Although, deploying low power base stations is an attractive solution to increase the overall network capacity and serve more users, the actual large scale deployment challenges, especially in terms of the backhaul and the availability of power line connections, are not very well investigated in the current literature. The lack of power line connections can severely limit the deployment of the new base stations at the locations where a new base station could have been most useful. Moreover, the reliability of power line connections is questionable in the areas where the power outages are common. Unreliable power line connections directly translates to mobile station outages, which degrades the Quality of Service (QoS) perceived by the mobile station.
Another reason that the demand of wireless data traffic is explosively increasing is the increasing popularity of smart phones and other mobile data devices such as tablets, netbooks and e-book readers among consumers and businesses. In order to meet the high growth in mobile data traffic, improvements in radio interface efficiency and most importantly allocation of new spectrum would be important.
The current fourth generation (4G) of mobile phone mobile communication technology standard systems including long term evolution (LTE) and Mobile Worldwide Interoperability for Microwave Access (WiMAX) use advanced technologies such as Orthogonal Frequency Division Multiplexing (OFDM), Multiple Input Multiple Output (MIMO), multi-user diversity, link adaptation, and the like, in order to achieve spectral efficiencies, which are close to theoretical limits in terms of bpx/Hz/cell (bit rate/frequency/cell). Improvements in air-interface performance introduce new techniques such as carrier aggregation, higher order MIMO, coordinated Multipoint (CoMP) transmission and relays, and the like. However, it is generally agreed that any further improvements in spectral efficiency will only be marginal at best.
When spectral efficiency in terms of bps/Hz/cell cannot be improved significantly, another possibility to increase capacity is to deploy many smaller cells. However, the number of small cells that can be deployed in a geographic area can be limited due to costs involved for acquiring the new site, installing the equipment and provisioning backhaul. In theory, to achieve 1,000-fold increase in capacity, the number of cells also needs to be increased by the same factor. Another drawback of very small cells is frequent handoffs which increase network signaling overhead and latency. Small cells are useful for future wireless networks, but are not alone expected to meet the capacity required to accommodate orders of magnitude increase in mobile data traffic demand in a cost effective manner.
Besides all these new technologies above, more is needed to meet the explosive demand of mobile data. On solution explored is to use millimeter-wave band (for example, 3-600 GHz spectrum) for Mobile Broadband (MMB) applications with wide area coverage. Key advantages for the millimeter-wave band frequencies are spectrum availability and small component sizes, such as small antennas and other small radio devices due to short wavelengths. The short wavelength of the small components enables more antennas to be packed in a relative small area, thus also enabling high-gain antenna in small form factor. Larger band can be used and much higher throughput can be achieved using MMB communications compared with the current 4G system. In current cellular system, most of the receivers have wide coverage, such as with an omni-antenna.